The present invention relates to a method and apparatus for positioning and mounting large installations on the seafloor or on mainland.
Examples of such positioning/mounting operations are the mating operation for a large offshore structure onto a previously mounted template on the seabed, or the mating of building-like structures onto a structure already standing on the ground, and where separately installed directing or docking piles are used. For reasons of simplicity, mainly seabed operations will be dealt with in the following.
A typical such mating operation may involve a supporting structure or "jacket" that is to be lowered down in order to be secured in a predetermined position on/above a template or an equivalent structure. The jacket shall be maneuvered downwards so that docking sleeves suitable for use therewith can be threaded down onto docking piles which have been driven or drilled down into the seafloor around the template, in order that the jacket may be placed exactly in position over the template. Two conflicting requirements are always presented in connection with mating operations of this kind.
On the one hand, there exists a requirement that the horizontal deviation between jacket and template be as small as possible in relation to the completely ideal position, this in order to secure the subsequent tieback operation, i.e. the coupling together of conductor strings to the already completed wells (well heads) in the template.
On the other hand, one would like to have large horizontal clearances or tolerances in the guiding system, i.e. between the docking piles and the guiding or docking cylinders which are to be threaded down onto the piles in order to simplify and secure the operation itself, which operation is critical regarding weather, wave and current conditions.
Traditionally a compromise is made, and one selected tolerances in the guiding system to make the mating operation feasible within a specified "weather window" based upon statistics on such conditions.
The previously known techniques regarding such positioning and mating is principally based upon three systems, all using two or more docking piles to effect guidance both "sideways and directionally" during the mating operation. As previously mentioned, these piles have been installed in advance. They have been driven or drilled down into the seabed under guidance from guide members mounted for instance on the ends of the previously mounted template. Then the jacket is floated or hoisted in above these docking piles.
A) "Fixed" system: When hoisting the jacket down, an open cylinder, the docking sleeve, which is fixed to the jacket, is firstly guided down onto the corresponding one of the docking piles. Thereafter the jacket is rotated until the other docking sleeves are in position right above their corresponding docking piles. Then, the jacket is lowered down the last part of the way, until the jacket is standing in its position on the seabed.
B) "Active" system: This system is similar to the "fixed" system above; however, the docking sleeves are mounted loosely each in a respective fixed cylinder, i.e. the docking sleeves may be moved vertically. The loose docking sleeves are suspended by respective wires, and thus may be lowered individually down onto the corresponding docking piles. In this system there is a possibility of raising the docking sleeves again if necessary.
C) "Passive" system: This system is also similar to the previous one; however, the loose docking sleeves are not suspended by wires and therefore cannot be re-raised.
The previously used systems are burdened with several drawbacks.
The ultimate and essential point is being able to achieve the joining together of the oil conductor strings belonging to the platform and the pipes protruding up through the template (i.e. the tieback operation). Because of the above-mentioned compromises, which must be made regarding tolerances in the dimensions of piles and sleeves as well as in the positions and angles thereof, angular deviations as well as deviations in position may become larger than advisable during the tieback operation. In other words, small tolerances for the tieback operation entail a greater risk when effecting the very critical mating operation, when using these prior art systems.
Moreover, with the known systems there exists a possibility that the parts may get stuck during the mating operation if the tolerances are exceeded. This may prove fatal.
As a secondary point, it should also be mentioned that the previously known sleeve system is always large and heavy, and in connection with these large steel structures it is also necessary to use corrosion preventing electrical systems of considerable size.